RFR: 8205064: Fail immediately if an unsupported GC is selected

[Roman Kennke]

Am 14.06.2018 um 22:34 schrieb Per Liden:
> If an unsupported GC (i.e. a GC that is not built into the VM) is
> selected by the user, the VM issues a warning and then continues and
> (silently) selects a different GC. Aleksey brought this up on the ZGC
> list [1]. I agree that this behavior seems dubious. With this patch we
> instead fail immediately to avoid unnecessary confusion.
> 
> Bug: https://bugs.openjdk.java.net/browse/JDK-8205064
> Webrev: http://cr.openjdk.java.net/~pliden/8205064/webrev.0
> 
> Testing: manual + currently running in mach5 t{1,2,3}
> 
> /Per
> 
> [1] http://mail.openjdk.java.net/pipermail/zgc-dev/2018-June/000422.html

Hi Per,

I am not sure I'd ever set Epsilon as default, but it's the last in
line, i.e. selected when built *only* with Epsilon, is that right?

Why not include CMS in that list? If I build with CMS and Epsilon, I get
Epsilon selected?

Other than that, I think the patch is good.

Thanks, Roman

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